Electrochemical gas sensors are well known and are extensively used for the monitoring of various gases in an atmosphere, particularly toxic gases such as carbon monoxide and hydrogen sulphide, and also for the detection of oxygen.
Electrochemical gas sensors include the following components:    (a) a gas-permeable working (or sensing) electrode, where the target gas is either oxidised or reduced electrochemically in a half reaction; the electrode is normally made from a semi-permeable gas diffusion membrane, typically PTFE, having a layer of catalyst deposited on one of its surfaces;    (b) a counter electrode at which an electrochemical half-reaction takes place to balance the electrons generated or consumed by the half-reaction taking place at the working electrode; and    (c) a body of electrolyte in contact with both the working and counter electrodes.
The sensor is such that gas from the atmosphere being monitored is in contact with the working electrode.
The sensor may additionally include a reference electrode in contact with the electrolyte to define a stable potential that the working and counter electrodes can be referenced against.
In some electrochemical sensors, generally those sensors that detect target gases that can be tolerated in the atmosphere in a relatively large quantity, e.g. carbon monoxide, the sensor may include a gas access port that limits access of gas to the working electrode. The gas access port may be provided in a cap that forms the top of the gas sensor. For example, for a CO sensor cell detecting up to 1000 ppm, a gas access port of approximately 1 mm diameter can be used.
In other sensors, generally those sensors for poisonous target gases that can only be tolerated in the atmosphere in a relatively small quantity, e.g. arsine at 200 ppb, it is not desired to restrict the access of the atmosphere to the working electrode and a wide gas access port or no gas access port at all is provided so that the working electrode is essentially in direct contact with the atmosphere. Such sensors are advantageous in having a fast response time since the atmospheric gases need not diffuse through a gas access port in order to reach the working electrode.
The working electrode is typically a semi-permeable, flexible PTFE membrane having catalyst applied to the membrane surface in contact with the electrolyte. The working, counter and, if used, reference electrodes each needs to be maintained in intimate contact with the electrolyte and the catalytic surfaces of the electrodes should not be allowed to become uncovered otherwise unexpected current/voltage characteristics may occur. In practice, this condition is achieved by placing a porous material between at least the working and the counter electrodes, ensuring that sufficient electrolyte is maintained in the porous material between the various electrodes and compressing the electrode assembly, i.e. the electrodes and the intervening porous material, together.
Where the sensor includes a cap with a gas access port, this compressive force can be achieved by pressing the electrode assembly into a sensor housing by means of the cap, which is generally relatively rigid. The cap presses down on the electrode assembly either directly or indirectly via a porous mat. For sensor cells having a relatively small opening (e.g. a CO sensor with a 1 mm diameter port), the gas access port does not cause any problem with ensuring compression in the cell. However, for those gases (e.g. arsine), requiring a large gas access port, the port can almost be as large as the whole cap or indeed a cap may be dispensed with altogether. In this instance, even if a cap is provided, it can, at best, compress only the outer rim of the electrode assembly, causing the thin porous membrane of the working electrode, which is placed immediately under the cap, to bulge, stretch and, in extreme cases, even to split. In order to prevent this, it is necessary to use a rigid or semi-rigid support structure in the sensor that presses down on the electrode assembly independently of the cap. Generally, such support structures are made of moulded plastic that are at least 1 mm in thickness. In one commercially available electrochemical gas sensor, the structure takes the form of a cross with arms at least 1 mm deep and at least 1 mm wide, which forms part of the top or cap of the sensor cell. During the assembly of the sensor, the top or cap, with the cross, is bonded to the main body of the sensor such that the cross maintains the electrode assembly in compression.
We have discovered that, by replacing the cross-shaped frame with a thinner rigid or semi-rigid structure for supporting the working electrodes, several advantages can be obtained and previously unappreciated disadvantages avoided.
WO02/073177 discloses an electrochemical gas sensor having a working and counter electrode and an intervening body of electrolyte absorbed in a separator. A reservoir of electrolyte is also present that is absorbed in a wicking material that supplies electrolyte to the separator, thereby keeping the separator saturated with electrolyte. A cap with a gas access hole compresses the electrodes, the separator and the wicking material.
GB 2287791 discloses an electrochemical gas sensor having a working electrode, a counter electrode and a rigid porous ceramic body between the electrodes that is flooded with electrolyte. The working electrode is a rigid stainless steel gauze onto which a catalyst is fixed; its outer rim is bent over to fit in with other components of the sensor. A gas permeable membrane made of polymer resin it placed on top of the working electrode and finally a filter is provided made of PTFE coated stainless steel. The electrode and the porous ceramic body between the electrodes are rigid and so the whole assembly does not need to be supported. It is important for the filter to have a relatively closed structure so as to ensure that incoming gases come into contact with it and so are subject to its filtering action.
WO96/04550 describes an electrochemical gas sensor having a working electrode covered by a selective membrane that removes cross-sensitive gases, i.e. gases that the working electrode will react with but are not the gases that are being detected. In order to work, the selective membrane must be gas permeable and, if porous, will have only small holes through it.